This invention relates generally to semiconductor lasers and, more particularly, to semiconductor laser diodes for applications requiring very low noise, single frequency operation. Laser diodes of this type are useful in such applications as coherent optical communication systems using either digital or radio-frequency analog technology, or in solid state pumping applications where precise wavelength control is required.
Basically, a semiconductor laser diode comprises an active semiconductor layer located at a diode junction, cladding layers surrounding the active layer in a sandwich-like fashion, and end mirrors of some kind located at opposite ends of the active layer. Photons generated in the active layer are substantially confined to the active layer by the surrounding cladding layers and are repeatedly reflected back and forth between the end mirrors. Laser light is emitted through one or both of the mirrors, in an edge emitting device, or in other designs there may be an inclined mirror to provide surface emission of the light. The nature of the laser cavity defined by the active layer and the mirrors is such that, in general, lasing may occur at more than one wavelength or mode of operation. Since sufficient gain for lasing may be typically achieved over a relatively wide bandwidth, the spectrum of light output from a conventional semiconductor laser diode consists of a number of peaks at various wavelengths. The multiple modes are less pronounced in continuous-wave (CW) operation than in pulsed operation, but are still present.
An early attempt to address this problem was a device known as the cleaved coupled cavity. To make this device, a semiconductor laser diode is cleaved into two parts, which are positioned side by side and optically coupled together in operation. Because the spectral positions of the multiple modes in each half of the device are not identical and overlapping, they tend to be eliminated from the spectrum of the pair operating together. By appropriate design, there is one mode that appears identically in both halves of the device. The cleaved coupled cavity has not, however, been made to operate reliably.
Another approach to attaining single-wavelength operation is to use feedback from a reflection grating located external to the laser cavity. If the grating is designed to reflect light only in an appropriately narrow bandwidth, this will confine lasing to a single desired longitudinal mode of operation. The difficulty is that, although this concept works well in a laboratory setting, it is difficult to build into a practical device. The grating has to be located approximately 10 cm from the laser diode and must, of course, be accurately position and aligned. Thus, the resulting device is highly sensitive to mechanical shock or vibration, and is much more bulky. The semiconductor laser diode itself is only a millimeter or so in length, but the grating must be positioned a hundred times that distance away from the laser. For these reasons, the use of grating feedback to obtain single frequency operation is not considered a practical solution.
Some designs have attempted to integrate grating reflectors onto the same substrate as the laser diode, but such structures are difficult to fabricate. A relatively low yield of devices is obtained from the fabrication process and the resulting devices are not reliable.
Another solution is to extend the laser cavity outside the semiconductor structure, using external mirrors to define the cavity instead of reflective cleaved facets of the semiconductor material. Then an etalon may be installed in the cavity, i.e. between the external mirrors, but outside the semiconductor structure defining the laser gain region. This approach has the same drawbacks as the grating feedback device, i.e., the device is more bulky and difficult to maintain in alignment.
Other attempts to integrate etalons into laser cavities include those described in U.S. Pat. No. 5,185,754 to Craig et al. and in U.S. Pat. No. 4,815,084 to Scifres et al. The Craig et al. patent discloses a waveguide structure in which an etalon is introduced by etching a channel through the waveguide and the Scifres et al. patent discloses a waveguide structure in which the effect of an etalon is achieved by means of changes in the effective index of refraction in a waveguide. A change in effective index is limited to less than 1%, which yields a mirror reflectivity of the order of only 2.5.times.10.sup.-5. Etching through a waveguide yields a pair of reflective mirrors having reflectivities of approximately 30%. The effective reflectivity of such a pair of mirrors depends strongly on the device geometry and is, at best, about a 46% reflectivity. In addition to not providing high reflectivity, neither of these techniques affords precise control of the resulting etalon modes of oscillation, which is needed to control the wavelength of operation.
Accordingly, it will be appreciated that there is a need for further improvement in the field of semiconductor laser diodes. In particular, what is needed is a semiconductor laser diode having a basically single frequency of operation, but in a compact and rugged device that does not need to be maintained in alignment. The present invention satisfies this need.